This invention relates to a semiconductor light emitting element, its manufacturing method and a semiconductor light emitting device, especially to a semiconductor light emitting diode in which a light is emitted by injecting a current into a p-n junction and the light can be extracted with a higher efficiency, its manufacturing method and a semiconductor light emitting device using the light emitting diode.
A semiconductor light emitting device has been widely used due to the advantages such as a lower consumption power, a higher efficiency and higher reliability in a various kinds of display devices and LCD back-light instead of an incandescent bulb. Especially, since InGaAlP based material has a direct transition property, the light of which wavelength is between red and green is obtained with a higher efficiency. Therefore demands are expanding for a red stop lamp for a car, a yellow turn signal lamp for a car, and a red and yellow lamp for a traffic signal.
A basic structure of the semiconductor light emitting device is explained below. A light emitting device emits a spontaneous light by the recombination of electron-hole pairs in an active layer when a current is injected into a p-n junction. In general, the light emitting element in a chip form is mounted on a lead frame having a reflecting metal and is encapsulated in an epoxy resin having a lens function. A part of the emitted light propagates through the lens-like package directly, and another part of the emitted light is reflected by the reflecting metal and then propagates outward through the lens-like package. The structure of the chip will be described below.
A GaAs substrate is used for growth of an InGaAlP based multi-layer. However since the GaAs substrate absorbs the light emitted from InGaAlP based material, another method has been introduced by which the InGaAlP based multi-layer is formed on the GaAS substrate, the surface of the multi-layer is bonded to a GaP substrate and the GaAs substrate is removed finally. The GaP does not absorb the light from InGaAlP based material and hence is transparent substantially.
Although the device fabricated by the above-mentioned method can reduce the absorption to some extent, the light extraction efficiency is not high satisfactorily. This is because a part of the light which is emitted toward all directions (360°) is reflected at an interface between the semiconductor and the epoxy resin package due to the refractive index difference, and can not be extracted externally. For example, if the refractive index of the semiconductor is 3.3 and that of the epoxy resin is 1.5, then a critical angle θc becomes 27°(θ=sin−1(1.5/3.3)=27°) according to Snell's law.
Therefore, when the light enters into the epoxy resin with an incident angle greater than 27° from the semiconductor, the total reflection occurs and can not be extracted. Since the light emitting device has a hexahedron shape generally, the extraction efficiency may be 28% approximately for ideal case. However the extraction efficiency becomes less than 28%, because an n-side and p-side electrode are provided on both surface, one electrode is mounted on the lead frame or the like with an adhesion bond, and the incident light into the electrode is absorbed in the alloyed layer of the electrode.
In order to solve the above problem, a transparent substrate structure having an appropriately designed shape is disclosed in the Japanese Patent Laid-Open No. 10-341035, for example.
By appropriately forming the shape of the transparent substrate, the light emitted from the active layer is reflected toward the light extraction direction. Consequently, a lot of light can be extracted outside.
The major problems of the above-mentioned device are the poor production yield and the shorter life time of the device. For example, when the device is mounted on a lead frame using a conductive adhesion bond, the assembling yield tends to fall because of the shortage between an edge of the p-n junction and the surface of the n-side electrode. If the amount of the adhesion bond is reduced so as to avoid a contact between the p-n junction and the adhesion bond, the adhesion strength becomes reduced and the device chip tends to fall off from the lead frame during a long operation period. It causes a shorter life time problem.